1. Field of the Invention
The invention pertains to semiconductor devices having a Schottky-barrier contact and in particular to such devices constructed for operation in the microwave frequency ranges in which a stable contact is desired.
2. Description of the Prior Art
In prior art microwave diodes and field effect transistors employing Schottky-barrier contacts, the contact was ordinarily formed by a layer of platinum or other Schottky-barrier forming metal deposited upon a semiconductor surface in a desired pattern with a layer of highly conductive metal such as gold deposited over the platinum layer to provide external contact. The platinum layer was made thick enough to prevent atoms of the gold layer from migrating through pinholes or grain boundaries in the platinum layer into the semiconductor material and, by their presence in the semiconductor material, degrading the performance of the device. Unfortunately, the excess platinum over that which was required to form the Schottky-barrier junction allowed for continuing alloying between the platinum and semiconductor material during high temperature conditioning and operation of the device. The position of the Schottky barrier effectively moved further into the semiconductor material as the alloying process continued. With many devices such as Read-type IMPATT diodes, such movement is quite undesirable as the position of the Schottky-barrier junction determines one boundary of the avalanche or charge carrier creation region. In some configurations of such devices, the frequency of operation of the device was changed as the junction moved.